style: scroll-snap-coordinate shouldn't use NotInitial.

The initial value for this is indeed `none` (and thus empty). The Rust code was
confused.

This property is disabled by default these days, and I think the
get_initial_value() function, which is what could get confused, is not called
for this property, so I think this shouldn't be observable.

Differential Revision: https://phabricator.services.mozilla.com/D30124

components/style/properties/longhands/box.mako.rs

@@ -337,11 +337,11 @@ ${helpers.predefined_type(
     "Position",
     "computed::Position::zero()",
     vector=True,
+    allow_empty=True,
     products="gecko",
     gecko_pref="layout.css.scroll-snap.enabled",
     spec="Nonstandard (https://developer.mozilla.org/en-US/docs/Web/CSS/scroll-snap-destination)",
     animation_value_type="discrete",
-    allow_empty="NotInitial",
 )}
 
 <% transform_extra_prefixes = "moz:layout.css.prefixes.transforms webkit" %>